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Baseboard Calculation

HVAC tech came today as we will be converting from oil to NG in a few weeks. In addition to the boiler conversion we are adding "supplemental" heat to our bonus room above the garage. I have already insulated the knee walls but not much help. Can't afford to redo the ceiling of the garage. However, I am concerned about his calculation on how much baseboard we need and am looking for some feedback.

The room does have heat however there is a 15 degree differential btwn the main part of the house and the bonus room. The bonus room does have its own return. We typically keep the door closed all winter b/c it does have its own return and if left open really just disperses the cold air into the upstairs area.

HVAC tech said 23ft of baseboard at 500btu/hr/lf would be more than adequate.

1) with the current system, it is almost as if there is no heat b/c it is so cold (typically 50 degrees all winter long). Should he be addressing the room as if there is no heat being supplied in there?

2) should we ask him to install 23' of the SF Multipak80 baseboard that can have an output btwn 660-900? (I think)

3) We are fine with the room being heated at 65, so how much baseboard would we need to get it to 65 from 50 degrees? I don't want to have to set the t-stat to 80 to get 65. The room is not used on a daily basis....or it hasn't been b/c it is so darn cold in the winter.

Yes, understand that is a huge problem with builders. Since the pitch of the roof over the garage is lower than the pitch of the roof over the main house, we cannot even access the "attic" over the bonus room to add more insulation. Although I do realize the ceiling of the garage is also an issue. So how and what do I tell the HVAC tech and how much baseboard do we need?

Yes, understand that is a huge problem with builders. Since the pitch of the roof over the garage is lower than the pitch of the roof over the main house, we cannot even access the "attic" over the bonus room to add more insulation. Although I do realize the ceiling of the garage is also an issue. So how and what do I tell the HVAC tech and how much baseboard do we need?

It is hard to even give you a reasonabe idea without a heat loss load on that room. Does he have it layd out for baseboard on the three exterior walls?